Tubes used in sub-sea or underwater applications may have positive buoyancy, and thus may float unless weighted down. While in some situations positive buoyancy may be desirable, positive buoyancy can affect the performance of the tube and can also lead to an increase in collision-related accidents or damage from weather and wave action. Problems associated with positive buoyancy may be particularly acute with tubes that are manufactured from low-density structural materials or have large volumes of bore relative to the volume and density of the structural material.
The buoyancy of a tube generally depends upon the density of the tube, the size of the tube, and density of the fluids located inside and outside of the tubing. Buoyancy may also depend on the ratio of the inner diameter of the tubing to the cross-sectional area of the structural wall of the tubing. Tubes made from low-density materials, such as composite tubing, may have positive buoyancy when used to transport gasses, or fluids having a high concentration of gasses contained within the fluid.
In many situations in which a low-density material pipeline is installed underwater, or sub-sea, a weighting system has to be employed to weigh down the tubing forming the pipeline. Typically, discrete weights that may be made from concrete or metallic materials are employed to weigh down the pipeline to achieve overall negative buoyancy for the pipeline. Discrete weighting systems, however, have significant disadvantages. First, discrete weighting systems are generally time-consuming to install. Secondly, underwater personnel are often needed to physically attach a plurality of discrete weights to the pipeline. This can be expensive, and it unnecessarily exposes the personnel to the risks and dangers inherent in such underwater operations. Additionally, discrete weighting systems often do not lend themselves to shallow water installations. For example, when discrete weighting systems are used in shallow water installations, the inherently positive buoyant pipe can form catenaries between the discrete weights. This can result in sections of the pipeline rising above the sea-bed, or even reaching the surface, where the pipeline can be easily damaged by surface activities.
A further disadvantage of using discrete weighting systems is that these systems are static systems, such that that the weights cannot quickly and easily be moved from one location to another. Thus, in discrete weighting systems the location of the placed weights typically cannot be changed in real-time, or near real-time, in response to possible changes in the operational or environmental conditions.
Sub-sea or underwater applications of tubing include installations in which the tubing is installed on the seabed or trenched beneath the seabed. Such installations are generally static installations, as the installed pipeline is typically exposed to generally static loads only. In other applications, the tubing can be installed so that it traverses the water column from one depth to another, for example, from the seabed to surface. Such applications are generally considered dynamic installations, and are commonly found in the oil and gas offshore industry, as for example, pipe systems used in the production of oil or gas that traverse from a sub-sea well head to the surface as a riser, flow line, control line, or umbilical line. A piping system of low density tubing used in a dynamic installation may be subjected to dynamic loads caused by the changes in water depth, internal or external pressures, relative motion of the surface termination of the piping system compared to the sub-sea termination, currents, or other loadings. These shifting dynamic loads can adversely affect a piping system's performance capabilities. Typically, it is not possible to change the dynamic response or behavior of the piping system when discrete weighting systems are used.
For these reasons there is a need for buoyancy control systems that are easy to use and install, that may be static or dynamically controllable, and that may be installed without the use of underwater personnel.